Magnetic resonance imaging device

ABSTRACT

In a contrast MRA measurement, sampling order of k-space is controlled considering the distance from the origin such that sampling the low-frequency data is performed a time when the contrast concentration reaches it peak. First, the sampling points of k-space are divided into two groups. Then, a measurement of the first group is started a time when the contrast concentration of a blood vessel of interest becomes high and is controlled from the high-frequency component to the low-frequency component such that the distance of a sampling point from the origin progressively decreases. A measurement of the other group, which is performed successively, is controlled from the low-frequency component to the high-frequency component such that the distance of a sampling point from the origin progressively increases. According to this ordering, influence of measurement time error in the contrast MRA measurement can be reduced and the whole blood vessel can be imaged with high contrast. In addition, an artery can be selectively imaged.

FIELD OF THE INVENTION

[0001] The present invention relates to a magnetic resonance imagingapparatus (MRI apparatus) for obtaining tomograms of desired portions ofan object to be examined using nuclear magnetic resonance (NMR). Inparticular, it relate to an MRI apparatus capable of obtaining desiredrange of images of excellent quality in minimal time to enablevisualization of movement in the vascular system.

BACKGROUND OF THE INVENTION

[0002] An MRI apparatus utilizes NMR to measure density distribution andrelaxation time etc. of atomic nuclear spins (referred as spinshereinafter) in a desired portion of an object to be examined anddisplays images of desired slices of the object produced from themeasured data. Conventional MRI apparatuses have a blood flow imagingfunction called MR angiography (MRA). This function includes a methodusing a contrast agent and a method using no contrast agent.

[0003] In a common method using a contrast agent, a gradient echo typesequence of short TR (repetition time) is used in combination with aT1-shortening type contrast agent such as Gd-DTPA. The method utilizesthe fact that blood spins containing T1 shortening contrast agent arenot likely to be saturated by the repeated excitation of a short TRbecause the blood spins have a shorter T1 than those of surroundingtissues and generate high-strength signals relative to the surroundingtissues, and enables to visualize blood vessels filled with bloodcontaining a contrast agent with high contrast relative to the othertissues. Measurement of volume data including blood vessels, typicallythree-dimensional measurement, is conducted while the contrast agentremains in the blood of interest and the obtained three-dimensionalimages are combined and subjected to projection process to depict bloodflow. In order to obtain information of wide range and high resolution,sequence based on the three-dimensional gradient echo method forobtaining three-dimensional data is generally employed.

[0004] In order to produce good images in such a three-dimensionalcontrast MRA, the following factors are important. (1) A manner ofinjecting a contrast agent, and (2) Measurement time or timing. Withregard to (1), the contrast agent should be injected such that its highconcentration is maintained in a blood vessel of interest in a stablemanner. For this purpose, a rapid injection method using an automaticinjection machine is generally utilized.

[0005] With regard to (2), if arteries are selectively imaged, forexample, the imaging timing is determined such that the concentration ofthe contrast agent becomes high when data are acquired. Ideally, theconcentration of the contrast agent reaches its peak value when acentral part (low frequency region) of the k-space which controls theimage contrast is sampled. The timing is determined corresponding todate-acquisition ordering of an employed pulse sequence.

[0006] Conventional data-acquisition ordering includes a sequentialordering in which data are acquired from one high-frequency side ofk-space toward the other high-frequency side via a low-frequency region,and a centric ordering in which data are acquired from low-frequencyregion of the k-space toward both high-frequency sides alternately.Generally, the centric ordering is employed. In a centric ordering in athree-dimensional measurement, one of a phase encoding loop and a sliceencoding loop is set to be an outer loop and the other to be an innerloop, and either or both is controlled by a centric ordering.

[0007] However, as shown in FIG. 1(b), the centric ordering in this caseis not a centric ordering in a true sense because a distance between thek-space origin and a sampling point fluctuates, and therefore likely tobe influenced by movement of an examined object and makes separation ofarteries and veins insufficient.

[0008] For solving this problem, an elliptical centric ordering isproposed, in which sampling order is controlled in view of the relativeFOV (Field of View) such that the distance from the ky-kz space originto a sampling point increases as the measurement proceeds (FIG. 1(c))(“Performance of an Elliptical Centric View Order for Signal Enhancementand Motion Artifact Suppression in Breath-hold Three-DimensionalGradient Echo Imaging. Alan, et al. Magnetic Resonance in Medicine38:793-802,1997”)

[0009] This data-acquisition method enables to produce arterial imagesselectively by starting measurement at the time when the concentrationof a contrast agent in the blood vessel of interest increases sincelow-frequency region data that dominates the image contrast are measuredat the beginning of the measurement time.

[0010] Although the above-mentioned centric ordering and ellipticalcentric ordering enable to determine the image contrast in the earlystage of measurement and are efficient for obtaining arterial images, ifthe optimal measuring moment is missed, the low-frequency information isacquired during the concentration of a contrast agent is low and therebyimage quality becomes degraded. Especially, if the measuring time is tooearly, the low-frequency data is sampled in the time period when signalsof a blood vessel are extremely low and the high-frequency region datais sampled in the time period when the signals of the blood vessel arehigh. This causes rinsing artifacts having no direct current component.In addition, the measurement time is prolonged because overallmeasurement is performed from the origin as a center toward thehigh-frequency region of the k-space horizontally or vertically.

[0011] On the other hand, the sequential ordering enables to producestable images in which remarkable artifact is not likely to generateeven if the measurement timing is somewhat wrong. However, this orderingis susceptible to movement of an examined object similarly to theaforementioned centric ordering and separation of artery and veinsbecomes insufficient.

[0012] Accordingly, an object of the present invention is to provide anMRI apparatus capable of visualizing a whole of a blood vessel ofinterest with high contrast in minimal time while reducing the influenceof time shift (error) from an optimal measurement time. Another objectof the present invention is to provide an MRI apparatus which isinsusceptible to the influence of movement of an examined object andcapable of visualizing arteries and veins separately. Yet another objectof the present invention is to provide a data-acquisition methodsuitable for MRA.

DISCLOSURE OF THE INVENTION

[0013] In order to achieve the above-mentioned objects, an MRI apparatusof the present invention employs a data-acquisition method in whichsampling points of k-space are divided into two groups and, in the firstgroup which is measured first, sampling order is controlled from thehigh-frequency region toward the low-frequency region such that thedistance from the k-space origin to a sampling point progressivelydecreases and, in the second group, sampling order is controlled in anopposite manner from the low-frequency region toward the high-frequencyregion such that the distance from the k-space origin to a samplingpoint progressively increases.

[0014] Specifically, an MRI apparatus of the present invention comprisesstatic magnetic field generating means for generating a static magneticfield in a space where an object to be examined is placed, gradientmagnetic field generating means for applying gradient magnetic fields inthe slice direction, phase encoding direction and readout direction,transmitting means for applying high-frequency magnetic field to causenuclear magnetic resonance in atomic nuclei of a living tissue of theobject, receiving means for detecting echo signals emitted by thenuclear magnetic resonance, control means for controlling the magneticfield generating means, transmitting means and receiving means, signalprocessing means for performing image reconstruction operation using theecho signals detected by the receiving means, display means fordisplaying the produced image, wherein the control means performs athree-dimensional sequence including a slice encoding step and a phaseencoding step, upon performing the sequence, divides sampling points ofa k-space defined by a slice encode number and phase encode number intotwo groups and controls the gradient magnetic field generation means ofslice direction and phase direction such that the distance from theorigin of the k-space to a sampling point progressively decreases in themeasurement of the first group and the distance from the origin of thek-space to a sampling point progressively increases in the measurementof the second group.

[0015] The manner of dividing the sampling points of the k-space intotwo group may be such that at least one of the groups includes samplingpoints from a low-frequency region to a high-frequency region and theother includes at least sampling points of a low-frequency region. Anumber of sampling points which are really measured may be the same ordifferent in the two groups. Namely, either of the two groups mayinclude non-sampling points (the points which are not measured).

[0016] Specifically, one embodiment is that sampling points are dividedinto two groups depending on the region of the k-space. According tothis embodiment, the control system performs a three-dimensionalsequence including a slice encoding step and a phase encoding step, uponperforming the sequence, divides the k-space defined by the slice encodenumber and the phase encode number into two regions, and controlsgradient magnetic field generating means of the slice direction and thephase direction such that, in one of the regions, the distance from theorigin of the k-space to a sampling point progressively decreases and,in the other region, the distance from the origin of the k-space to asampling point progressively increases.

[0017] In another embodiment, the sampling points of the k-space aredivided into two groups which are in a relation of complex conjugate.According to this embodiment, the control system performs athree-dimensional sequence including a slice encoding step and a phaseencoding step, upon performing the sequence, divides sampling points ofthe k-space defined by a slice encode number and phase encode numberinto two groups which share the origin and are in a relation of complexconjugate, and controls gradient magnetic field generating means of theslice direction and phase direction so that, in one of the groups, thedistance from the origin of the k-space to a sampling pointprogressively decreases and, in the other group, the distance from theorigin of the k-space to a sampling point progressively increases.

[0018] In this embodiment, it is preferable that adjacent two samplingpoints belong to different groups. In order to satisfy the conditionthat sampling points of the two groups are complex conjugate each other,some of the adjacent sampling points near the origin are required tobelong the same group. Accordingly, in the specification, the wording“to divide such that the adjacent sampling points belong to thedifferent groups” means the state satisfying as much as possible thecondition that the two groups are in a relation of complex conjugate andthe adjacent sampling points belong to the different groups.

[0019] In yet another embodiment of the MRI apparatus of the presentinvention, the control system does not measure all of the samplingpoints in one of the two divided regions but measures a smaller numberof sampling points than that of the other region. Or the control systemdoes not measure all of the sampling points in one of the two groups butmeasures a smaller number of sampling points than that of the othergroup.

[0020] A three-dimensional image data-acquisition method of the presentinvention comprises, when a three-dimensional image data is acquired byrepeating a plurality of times a step comprising selective excitation ofa predetermined region of an object to be examined, application ofencoding gradient magnetic fields at least in two directions, andcollection of echo signals generated from the region while changing theintensities of the gradient magnetic fields,

[0021] divides a measurement space defined by the gradient magneticfields encoding in the two directions, and

[0022] performs measurement of the tow divided regions sequentially suchthat the distance from the origin of the measurement space to a samplingpoint gradually decreases in the measurement of a region which ismeasured first and the distance from the origin of the measurement spaceto a sampling point gradually increases in the measurement of the regionwhich is measured thereafter.

[0023] In the data-acquisition method of the present invention, whenthere are several sampling points whose distances from the origin arethe same, the nearest sampling point from the latest measured point inthe k-space is preferably measured next.

[0024] A three-dimensional image data-acquisition method of the presentinvention, when three-dimensional image data is acquired by repeating aplurality of times a step comprising selective excitation of apredetermined region of an object to be examined, application ofencoding gradient magnetic fields at least in two directions, andcollection of echo signals emitted from the region while changing theintensity of the gradient magnetic fields,

[0025] divides sampling points on a measurement space defined by theintensities of the encoding gradient magnetic fields of the twodirections into two groups such that the first group and the secondgroup share the origin and that adjacent and conjugating sampling pointsbelong to different groups, and

[0026] performs measurement of the first group and the second group inthis order such that, in a measurement of the first group, the distanceof a sampling point from the origin of the measurement spaceprogressively decreases and, in a measurement of the second group, thedistance of a sampling point from the origin of the measurement spaceprogressively increases.

[0027] According to the data-acquisition method of the presentinvention, as shown in FIG. 1(a), measurement can be performed withoutfluctuation of distance from the origin and a blood vessel concerned canbe visualized with high contrast by making the time of measuring thelowest frequency component coincide with the time when the contrastagent enhance the signal intensity of the blood vessel most. Inaddition, even the time of measuring the low-frequency component issomewhat shifted from the peak of the signal intensity, proper samplingof the low-frequency component can be assured and images are notdegraded.

[0028] For reference, variations of distance from the origin of thek-space in the conventional centric ordering, elliptical centricordering and sequential ordering are shown in FIG. 1(b)-(d).

[0029] According to a preferable embodiment of the data-acquisitionmethod of the present invention, a part of all of the sampling points ismeasured in the first group and all of the sampling points are measuredin the second group.

[0030] Since the two groups are in a relation of complex conjugate, evenif a part of one group is not measured, data which are not measured canbe estimated from data of the other group. Particularly when ameasurement of the first group is performed during the density of thecontrast agent is increasing to its peak, sampling of unnecessary datahaving low signal intensity is avoided and thereby images of goodquality can be obtained.

BRIEF EXPLANATION OF DRAWINGS

[0031]FIG. 1 shows exemplary views of a data-acquisition method employedby the MRI apparatus of the present invention and conventionaldata-acquisition methods.

[0032]FIG. 2 is an overall block diagram of the MRI apparatus of thepresent invention.

[0033]FIG. 3 shows an example of a pulse sequence of contrast MRAmeasurement performed by the MRI apparatus of the present invention.

[0034]FIG. 4 shows an example of a k-space data-acquisition methodaccording to the present invention.

[0035]FIG. 5 is an exemplary view of MRA measurement according to theMRI apparatus of the present invention.

[0036]FIG. 6 shows simulation results of MRA measurement according tothe MRI apparatus of the present invention and the conventional MRAmeasurement.

[0037]FIG. 7 shows another example of a k-space data-acquisitionordering according to the present invention.

[0038]FIG. 8 shows yet another example of a k-space data-acquisitionordering according to the present invention.

[0039]FIG. 9 shows an example of a k-space data-acquisition orderingaccording to the present invention.

[0040]FIG. 10 shows an exemplary view of MRA measurement according tothe MRI apparatus of the present invention.

[0041]FIG. 11 shows yet another example of a k-space data-acquisitionordering according to the present invention.

[0042]FIG. 12 shows an exemplary view of an image reconstruction methodusing the data-acquisition method of FIG. 10.

[0043]FIG. 13 shows a simulation result for evaluating the MRAmeasurement by the MRI apparatus of the present invention.

[0044]FIG. 14 shows simulation results of MRA measurement according tothe MRI apparatus of the present invention and the conventional MRAmeasurement.

[0045]FIG. 15 shows another example of a k-space data-acquisitionordering according to the present invention.

THE BEST MODE FOR CARRYING OUT THE PRESENT INVENTION

[0046] An embodiment of the present invention will be explained withreference to the attached drawings hereinafter.

[0047]FIG. 2 is an overall block diagram of the MRI apparatus of thepresent invention. The MRI apparatus is for obtaining tomogramsutilizing NMR and comprises a static magnetic field generating magnet 2,a gradient magnetic field generating unit 3, a sequencer 4, atransmitting unit 5, a receiving unit 6, a signal processing unit 7, anda central processing unit (CPU) 8.

[0048] The static magnet field generating magnet 2 generates a uniformstatic magnetic field around an object to be examined in a directionparallel or perpendicular to the body axis of the object and comprisesmeans for generating a static magnetic field in a space around theobject such as a permanent magnet, a resistive magnet or asuperconductive magnet.

[0049] The gradient magnet field generating unit 3 comprises gradientmagnetic field coils 9 wound in the direction of three axes, X,Y,Z, anda gradient magnetic field power supply 10 for driving the gradientmagnetic field coils. The gradient magnetic field power supply 10 isdriven according to instructions from the sequencer 4 and appliesgradient magnetic fields Gx, Gy and Gz in the direction of the threeaxes, X,Y and Z, to the object. A manner of applying these gradientmagnetic fields determines a slice or slab of the object to beselectively excited and defines position or sampling order of samplingpoints in a measurement space (k-space).

[0050] The sequencer 4 operates under the control of the CPU 8, andsends necessary instruction for collecting data for obtaining tomogramsof the object to the gradient magnet field generating unit 3, thetransmitting unit 5 and the receiving unit 6. The operation timing ofthe gradient magnet field generating unit 3, the transmitting unit 5 andthe receiving unit 6 controlled by the sequencer 4 is called a pulsesequence. Here, a sequence for imaging three-dimensional blood flow isemployed as one of sequences. The control of the sequencer 4 will beexplained in more detail later.

[0051] The transmitting unit 5 is for producing an FR magnetic field inorder to cause nuclear magnetic resonance of nuclei of atomsconstituting the living tissues of the object in accordance with the RFpulse transmitted from the sequencer 4, and comprises an FR oscillator11, a modulator 12, an RF amplifier 13 and an RF coil for transmission14 a. The transmitting unit 5 amplitude-modulates the RF pulses outputfrom the RF oscillator 11 by the modulator 12 in accordance withinstructions from the sequencer 4. The amplitude-modulated RF pulses areamplified by the RF amplifier 13 and supplied to the RF coil 14 alocated in the vicinity of the object 1 so that electromagnetic wavesare radiated onto the object 1.

[0052] The receiving unit 6 is for detecting echo signals (NMR signals)elicited through nuclear magnetic resonance of atomic nuclei of theliving tissues of the object 1, and comprises an RF coil 14 b forreceiving electromagnetic waves, an amplifier 15, a quadrature phasedetector 16 and an A/D converter 17. The electromagnetic waves of theobject responding to the electromagnetic waves radiated by the RF coil14 a of the transmitting unit is detected by the RF coil 14 b located inthe vicinity of the object 1. The detected echo signals are input intothe A/D converter 17 via the amplifier 15 and the quadrature phasedetector 16, converted into digital signals, which are sampled by thequadrature phase detector 16 with a timing instructed by the sequencer 4to become two-series data, and transferred to the signal processing unit7.

[0053] The signal processing unit 7 comprises the CPU 8, recording mediasuch as a magnetic disk 18 and magnetic tape 19, and a display unit 20such as a CRT. The CPU 8 performs operations onto echo signals (digitalsignals) input from the receiving unit 6 such as Fourier transform,calculation of a correction coefficient and image reconstruction, andthereby produces and displays a signal intensity distribution of acertain section or images produced by performing suitable arithmeticoperations on a plurality of signals as a tomogram on the display unit20. In FIG. 2, the RF coils 14 a, 14 b for transmission and receivingand the gradient magnetic field coils 9 are disposed within a magneticfield space of the static magnet field generating magnet 2 placed in aspace around the object 1.

[0054] A first embodiment of a blood flow imaging performed by the MRIapparatus according to the present invention will now be explained.

[0055] As mentioned previously, the sequencer 4 controls operationtimings of the gradient magnet field generating unit 3, transmittingunit 5 and receiving unit 6 according to the predetermined pulsesequence, here a three-dimensional MRA sequence. The pulse sequence,similarly to other sequences, is stored as a program in a memoryprovided in CPU8 and selected properly by a user corresponding to thepurpose of imaging to be performed. Specifically, if MRA measurementusing a contrast agent is selected through an input module of CPU 8, theCPU 8 controls the sequencer 4 to perform the three-dimensional MRAsequence.

[0056] This pulse sequence is, as shown in FIG. 3 for example, asequence based on the gradient echo method and is common in theconventional three-dimensional MRA sequence. Specifically, after aregion (slab) including a blood vessel of interest is excited byapplying a selecting gradient magnetic field Gs together with an RFpulse RF, a gradient magnetic field pulse Ge1 in the slice direction anda gradient magnetic field pulse Ge2 in the phase encode direction areapplied and then a readout gradient magnetic field Gr is applied whilereversing its polarity to measure echo signals. Procedures from applyingthe RF pulse RF to measurement of echo signals are repeated whilechanging the intensities of the gradient magnetic field Ge1 in the slicedirection and the gradient magnetic field Ge2 of the phase encodedirection with a predetermined repetition time TR to acquirethree-dimensional data.

[0057] The encode number of the slice direction and that of the phaseencode direction define an image resolution in the both directions andare predetermined in consideration of a measurement time or the like.For example, the encode number of the phase encode direction is set tobe 128, 256, etc. and that of the slice direction is 10-30. The encodenumbers of the slice direction and phase encode direction also definek-space (provided that the slice direction is z and phase encodedirection is y, ky-kz space). In the sequence shown in FIG. 3, a signalmeasured when the intensity of the slice direction gradient magneticfield Ge1 has a value of Gz and the intensity of the phase-encodedirection gradient magnetic field Ge2 has a value of Gy is disposed at apoint (ky,kz) of the k-space corresponding to Gy and Gz.

[0058] Although the three-dimensional MRA sequence exemplified in FIG. 3is common in the conventional MRA, data-acquisition method of thissequence employed in this embodiment is different from the conventionalcentric ordering or elliptical ordering. In this method, the ky-kz spaceis divided into two along with a ky-axis or kz-axis and, in one regionwhich is measured first, a sampling order is controlled from thehigh-frequency component toward the low-frequency component such that apoint whose distance from the origin is large is sampled first and apoint to be sampled becomes closer to the origin 0. Contrarily, in theother region, a sampling order is controlled from the low-frequencycomponent toward the high-frequency component such that the origin 0 orthe vicinity thereof is sampled first and a point to be sampled becomesmore distant from the origin 0.

[0059] In this case, if there are several points whose distances fromthe origin are same, the nearest point from the last sampled point inthe k-space is sampled next.

[0060] Namely, this data-acquisition method is defined by “AND” of twoconditions: 1) In one of the two divided regions, a measurement isstarted with the most distant point from the origin and then points tobe sampled are successively determined such that the distance from theorigin decreases and, in the other region, a measurement is started withthe origin or the vicinity of the origin and then points to be sampledare successively determined such that the distance from the originincreases. 2) Distance between the temporarily adjacent sampling pointsbecomes minimum.

[0061] Although the condition 2) is not indispensable in thedata-acquisition method of the present invention, when the spatialdistance between the temporarily adjacent sampling points is made asclose as possible, artifact is suppressed. For this purpose, instead ofsatisfying the above condition of a distance between two samplingpoints, a sampling point having the same ky value or the nearest kyvalue may be selected as a next sampling point. Further, a samplingpoint may be determined in consideration of not only relationship of thetwo sampling points but also relationship among a plurality of samplingpoints which are sampled next or afterward.

[0062] As a simplified example of the afore-mentioned data-acquisitionmethod, FIG. 4 shows a data-acquisition ordering of the k-space having5*9 matrix where the slice encode number is 5 and the phase encodenumber is 9. In the figure, circled numbers indicate data-collectionorder. In this example, the k-space is divided into two by kz axis and,in the lower region (E→C region), data are sampled from a point(number 1) marked “start” in a high-frequency region toward the origin(number 25) in numerical order and, in the upper region (C→E region),data are sampled from the origin toward the high-frequency region andended at a number 45. The distance Δz between the adjacent points in thek-space coordinate is 1/FOVz.

[0063] Next, an embodiment of a contrast MRA measurement performed bythe aforementioned MRI apparatus employing such a data-acquisitionmethod will be explained with reference to FIG. 5.

[0064] First, an object to be examined is placed in a measurement spacewithin the static magnet field magnet and an imaging region including ablood vessel of interest is determined. Then, a timing measurement isperformed by a test injection method. In this method, a small amount ofa contrast agent (about 1-2 ml) is injected as a test dose in order toobtain a time-signal curve of the object portion as shown in FIG. 5.Arriving time t1 of the contrast agent is found from this curve and atiming of the measurement is determined based on the result. Instead ofthe test injection method, a method where an ROI (Region of Interest) isset in a specific portion within a monitored region, variation of signalintensity of that portion is monitored and a measurement is startedautomatically at a time when the signal intensity exceeds apredetermined threshold value, or a method where a blood vessel ofinterest is monitored in real time by repeating a short time measurementand display called fluoroscopy, and a measurement is started at a timewhen the signal intensity is properly enhanced can be employed for thetiming measurement. However, the test injection method is preferredbecause it enables to determine measurement timing accurately by using acontrast agent prior to a main measurement.

[0065] After the timing measurement, a main measurement is performed asshown in FIG. 5(b). The main measurement may be performed only afterinjection of the contrast agent but preferably performed before andafter injection of the contrast agent. The measurements are carried outsuccessively before and after injection for the same slice or slabposition under the same condition.

[0066] The imaging sequence is a short TR sequence based on thethree-dimensional gradient echo method as shown in FIG. 3. In this case,since an imaging object is blood flow, a gradient magnetic field forrephasing a dephase caused by flow, i.e., gradient moment nulling may beadded to the sequence. However, this is not essential and a simplegradient echo is rather preferable in order to shorten the TR/TE.

[0067] Once the repetition time TR of the pulse sequence and a matrixsize (slice encode number and phase encode number) are determined, themeasurement time T is determined. Then starting time t2 (a time frominjection of a contrast agent to the beginning of measurement) isdetermined based on the arriving time t1 of the target blood vesselobtained in the aforementioned timing measurement such that data in alow-frequency region of the ky-kz space are sampled at a time when thecontrast agent arrives at the blood vessel.

[0068] First, a measurement of the E→C region shown in FIG. 4 isperformed and then a measurement of the C→E region is carried out. Inthis case, the sequencer 4 controls both a slice direction gradientmagnetic field pulse and a phase encode direction gradient magneticfield pulse so as to sample from a high-frequency component toward alow-frequency component sequentially in a region measured first (forexample E→C region), and sample from a low-frequency component toward ahigh-frequency component sequentially in the region measured thereafter(for example C→E region). In this control, as mentioned previously, adistance from the origin to a sampling point gradually decreases in thefirst region and increases in the second region.

[0069] According to this control, the low-frequency component of thek-space is sampled at a time when the contrast agent arrives at thetarget blood vessel and the signal intensity of the blood becomeshighest and thus images of the artery can be visualized with highcontrast. Since there remains a time for sampling a low-frequencycomponent on both sides of the arriving time t1 of the contrast agent asshown in FIG. 5(b), even if the arriving time in the main measurementslightly differs from that (t1) determined in the timing measurement dueto a slight change of conditions between the two measurement, highquality images can be obtained without degrading the quality.

[0070] Results of simulation of separation of artery-vein usingdifferent data-acquisition methods are shown in FIG. 6. The simulationwas conducted under condition of FOV:320, TR:10 ms, phase encodenumber:160, slice encode number:16, image matrix:256*16, and slicethickness:5 mm. Separation of artery-vein is expressed using a ratio ofsignal intensity of an artery and a vein.

[0071] As understood from the figure, a signal ratio of the artery andvein is larger in the data-acquisition method of the present inventionthan in the sequential ordering and elliptical centric ordering.Accordingly, even if there is an indistinguishable vein near an arteryof interest, only the artery can be depicted with high contrast.

[0072] After three-dimensional image data are thus acquired before andafter injection of the contrast agent, the image data are subtracted toproduce a three-dimensional data of only blood vessels. The subtractionmay be a complex subtraction performed between slices on the same sliceposition of the three-dimensional data. Subtraction of absolute valuesmay be employed. Such a method of deleting tissues other than bloodvessels using subtraction between images obtained before and afterinjection of contrast agent is a known method called 3D MR-DSA(DigitalSubtraction Angiography) and is not essential for the present invention.However, use of this method is preferable for imaging fine bloodvessels, which are otherwise difficult to be visualized withsufficiently high contrast to other tissues.

[0073] After the subtraction, the three-dimensional data may beprojected in an arbitrarily direction such as coronal direction,saggital direction, transversal direction or the like to be observed inthree dimensions. A known method of projection such as Maximum IntensityProjection may be employed.

[0074] The first embodiment of the present invention has been explainedusing the examples shown in FIGS. 4 and 5. Various modifications thereofcan be employed. For example, although a sequence according to thegradient echo method is exemplified as a three-dimensional MRA sequencein the aforementioned examples, an EPI (Echo Planer Imaging) in which aplurality number of echo signals are measured at a single excitation ora multi-shot type EPI can be employed.

[0075] The k-space is divided by the kz axis in the aforementionedexample but may be divided by the ky axis. Further, although the casethat data are acquired symmetrically in the divided regions has beenexplained, the regions to be sampled may be asymmetrical.

[0076] An example of the asymmetrical acquisition case will be explainedwith reference to FIG. 7. In this case too, k-space having a matrix sizeof 5*9 and divided by the kz axis is exemplified. In the asymmetric dataacquisition, a measurement is started not from the high-frequency regionbut from the middle frequency region (number 1) in the first region,e.g., E-C region, toward low-frequency region (number 15) in numericalorder. After that, the overall C→E region is sampled from low-frequencyregion to high-frequency region similarly to the case shown in FIG. 4.Alternatively, an overall region measured first is sampled and thelatter half region is sampled from low-frequency region tomiddle-frequency region. Referring to the example shown in FIG. 7, ameasurement is started with a number 35 and performed to a number 1 indecreasing numerical order.

[0077] By making the number of sampling points in one of the two regionsless than in the other region, a total measurement time can beshortened. Specifically, as shown in FIG. 5(c) and (d), by reducing thenumber of sampling points in a region to be measured first or later, themeasurement time becomes shorter than the case shown in FIG. 5(b).Further, a measurement method shown in FIG. 5(d) is effective in a casethat the distance between a blood vessel of interest and a neighboringvein is short, and interval between an arrival time of the contrastagent at the blood vessel of interest and an arrival time at theneighboring vein is short. In this case, when the posterior region issampled from the low-frequency component toward the middle-frequencycomponent, signals from the neighboring vein is kept from getting mixed(a dot line signal intensity curve).

[0078] In this case, data which have not been sampled of the regionhaving a smaller number of sampling points (data of the region shown byoblique lines) may be estimated from data of the other region which havebeen sampled, or may be filled with 0. In addition to the aforementionedadvantage, this exemplified method enables to obtain a selectivearterial image with high contrast while reducing influence of time shitof three-dimensional measurement.

[0079] k-Space data acquisition is not limited to the square matrix asshown in FIGS. 4 and 7 but only data within a circle (ellipse) centeringaround the origin (number 15) may be acquired as shown in FIG. 8. InFIG. 8, the slice encode number of 5 and the phase encode number of 9are also exemplified but, in this example, a region where both ky and kzbecome high-frequency component (outer side of the circle) is notsampled and data on concentric circles centering around the origin(number 15) are sampled. Data acquisition order is, as shown by circlednumbers, in such a manner that the lower half is sampled from a distantpoint from the origin toward the origin and the upper half is sampledfrom the origin toward distant point.

[0080] In this case, the same effect as that of the method illustratedin FIG. 4 can be obtained. In this case too, data may be compensatedusing measured data.

[0081] The first embodiment of the present invention, in which thek-space is divided into two regions and sampling points are divided intotwo groups corresponding to the region, has bee explained hitherto. Thesampling points may be divided into two groups regardless of the region.

[0082] The second embodiment of the present invention, in which thesampling points of the k-space are divided into two groups in such amanner that sampling points having a relationship of complex conjugatebelong different groups.

[0083] In this embodiment, when matrix points of sampling points of thek-space, e.g. ky-kz space, are to be divided into two, the followingconditions should be satisfied: 1) the two groups share the origin andthe sampling points of the two groups are in a relation of complexconjugates, 2) adjacent sampling points in the k-space belong todifferent groups. Then these two groups are sampled successively.

[0084] As a simplified example of data-acquisition method according tothe second embodiment, FIG. 9 shows k-space having a matrix of 8*8 witha slice encode number of 8 and a phase encode number of 8. This matrixhas sixty four of matrix points (sampling points), which are dividedinto the first group shown in the right side of the figure and thesecond group shown in the left side. The matrix points belonging to thetwo groups are in a relation of complex conjugate and the adjacentpoints belong to the different group. However, adjacent points near theorigin have to belong to one group in order to satisfy the relationshipof complex conjugate.

[0085] Sampling of the first group of two, which is sampled first, isstarted with a distant point from the origin 0 and is controlled fromthe high-frequency component to the low-frequency component such thatthe distance from the origin 0 to a sampling point becomes closerprogressively. Sampling of the second group is controlled in an oppositemanner from the low-frequency component to the high-frequency componentsuch that the origin 0 or the neighborhood is sampled first and thedistance from the origin 0 to a sampling point becomes more distantprogressively.

[0086] The circled numbers in the figure show the data acquisitionorder. Sampling points having the same number have no priority andeither may be sampled first.

[0087] In the first group to be sampled first, measurement is startedwith the most distant matrix point (number 1) from the origin (number 33is assigned), i.e., the highest-frequency component and proceeded to thematrix point of number 2 next, the matrix point of number 3 and so on innumerical order. Measurement of the second group is succeeded, in whichthe nearest point (number 34) from the origin, the low-frequencycomponent, is sampled first and the distance from the origin to a samplepoint increases progressively.

[0088] An example of a contrast MRA by the above MRI apparatus employingsuch a data-acquisition method now will be explained with reference toFIG. 10.

[0089] In the same manner as that of the first embodiment, an object tobe examined is placed in a measurement space within the static magnetfield magnet and a measurement region including a blood vessel ofinterest is determined. Then a timing measurement is carried out. Inthis case too, the timing measurement is performed using the testinjection method for example. Specifically, a small amount of a contrastagent (about 1-2 ml) is injected into the object and a time-signal curvein the target portion is obtained as shown in FIG. 10. Arrival time t1of the contrast (a time of signal intensity peak) is found from thecurve and timing of a main measurement is determined based on theresult.

[0090] After the timing measurement, the main measurement is performedas shown in FIG. 10(b). The main measurement may be carried out onlyafter injection of the contrast agent but preferably carried out beforeand after injection. The measurements are carried out successivelybefore and after injection for the same slice or slab position under thesame condition.

[0091] The imaging sequence is a short TR sequence based on thethree-dimensional gradient echo method as shown in FIG. 3. In this case,since an imaging object is blood flow, a gradient magnetic field forrephasing a dephase caused by flow, i.e., gradient moment nulling may beadded to the sequence. However, this is not essential and a simplegradient echo is rather preferable in order to shorten the TR/TE.

[0092] Once the repetition time TR of the pulse sequence, a matrix size(slice encode number and phase encode number) and a number of additionare determined, the measurement time T is determined. Then starting timet2 (a time from injection of a contrast agent to the beginning ofmeasurement) is determined based on the arriving time t1 at a bloodvessel of interest found in the aforementioned timing measurement suchthat a measurement of data in a low-frequency region of the ky-kz spaceare sampled at a time when the contrast agent arrives at the bloodvessel.

[0093] First, a measurement of the first group is started and then ameasurement of the second group is performed. In this case, thesequencer 4 controls both a slice direction gradient magnetic fieldpulse and a phase encode direction gradient magnetic field pulse so asto sample from a high-frequency component toward a low-frequencycomponent sequentially in a region measured first, and to sample from alow-frequency component to a high-frequency component progressively inthe region measured thereafter. Thus three-dimensional image data areobtained.

[0094] In the data-acquisition method illustrated in FIGS. 9 and 10, allof the sampling points are sampled for both of the first and secondgroups. However, as shown in FIG. 10(c), another data-acquisition methodmay be employed in which sampling of a predetermined high-frequencycomponent is omitted and a low-frequency component is sampled for ashort time.

[0095] An example of such a data-acquisition method is shown in FIG. 11.In the figure, the k-space having matrix size of 8*8, a slice encodenumber of 8 and phase encode number of 8, is illustrated. Althoughconditions of dividing k-space into two groups in this example is thesame as those of the example shown in FIG. 9, a predeterminedhigh-frequency component is not sampled and only low-frequency componentis sampled in the first group which is measured first. In theillustrated method, only matrix points on the 4*4 matrix among thematrix points of the k-space are sampled. Among these matrix points inthe low-frequency region, a most distant matrix points from the origin(point of number 9) is sampled first, and sampling is proceeded to apoint of number 2 next, a point of number 3 third and to the origin.

[0096] In the second group, similarly to the example shown in FIG. 9,sampling is started with a matrix point (number 10) adjacent to theorigin and proceeded toward the most high-frequency component in anorder according to the distance from the origin, and thus overall pointsbelonging to the second group is sampled.

[0097] In this case, high-frequency data which have not sampled in thefirst group are estimated from data based on the complex conjugaterelationship between the first group and the second group. A manner ofestimating the data which have not been sampled may be a known methodbased on a half-Fourier transformation. The procedure of the estimationis shown in FIG. 12. Sampled data are subjected to one-dimensionalFourier transform in a frequency encode direction (kx direction) toproduce a three-dimensional hybrid space sampled data, from whichthree-dimensional data is estimated. Then the estimated hybrid spacedata and combined with the sampled data to obtain a complete hybridspace data. Fourier transform of this complete hybrid space dataproduces three-dimensional image data. Employing the estimation of dataprevents degradation of special resolution even if the number of datapoints is reduced.

[0098] In this embodiment, similarly to the data-acquisition methodshown in FIG. 9, the low-frequency component of k-space is sampled whenthe contrast agent arrives at the blood vessel of interest and thesignal intensity of the blood vessel is enhanced most. Accordingly,images of an artery can be imaged with high contrast.

[0099] In addition, since the concentration of a contrast agent (signalintensity) rapidly increases after injection of the contrast agent asshown in FIG. 12, if this imaging method is performed so that a samplingtime of the most low-frequency component is synchronized with a signalintensity peak, sampling of unnecessary data before arrival of thecontrast agent is omitted because of its short measurement time beforethe peak.

[0100] Results of simulation of artery-vein separation using differentdata-acquisition methods are shown in FIGS. 13 and 14. The simulationwas conducted using an imitated artery and vein, through which acontrast agent is passed at a flow speed of 40 cm/s, a circulating timebetween the artery and vein of 7 seconds, and an injecting speed of 2cc/sm, and images of the artery and the vein were produced usingdifferent imaging methods. FIG. 13 shows signal intensities obtainedunder the above conditions. As shown in the figure, a peak intensity ofsignals from the artery is observed first and then a peak intensity ofsignals from the vein appears behind. FIG. 4(a) shows an image producedby the imaging method of the present invention and (b) shows an imageproduced by the elliptical centric ordering.

[0101] As understood from the figure, not only the artery but also thevein are imaged and these two blood vessels are not completely separatedby the elliptical centric ordering. On the other hand, only the arterycan be imaged with high contrast in the imaging method of the presentinvention.

[0102] Although a gradient echo sequence has been also exemplified as athree-dimensional MRA sequence in the second embodiment, an EPI (EchoPlaner Imaging) sequence which enables to sample a plurality of echosignals with one excitation or a multi-shot EPI may be also employed.

[0103] Further, the k-space data to be acquired are not limited to asquare matrix data as shown in FIGS. 9 and 11 may be data within acircle (ellipse) centering around the origin as shown in FIG. 15. In thefigure, the first group, which is indicated by a real line, is sampledfrom the high-frequency component toward the low-frequency componentsuch that a distant point from the k-space origin is sampled first andthe distance to the origin decreases. Contrarily, the second group issampled from the low-frequency component toward the high-frequencycomponent such that sampling is started with the k-space origin orneighborhood and the distance from the origin to a sampling pointincreases. The same effect as that of the method illustrated in FIGS. 9and 11 can be obtained and sampling of the high-frequency component ofthe first group can be omitted.

INDUSTRIAL APPLICABILITY

[0104] As explained hitherto, the MRI apparatus of the present inventionemploys a sampling control method in which ky-kz space matrix points(sampling points) are divided into two groups and the first group to besampled first is sampled from the high-frequency component toward thelow-frequency component such that the distance of a sampling point tothe k-space origin progressively decreases and the second group issampled from the low-frequency component toward the high-frequencycomponent such that the distance of a sampling point to the k-spaceorigin progressively increases, thereby reduces influence of imagingtiming error and provides high contrast images in which a goodartery-vein separation is achieved. In addition, by reducing a number ofsampling points of one of the groups, a measurement time can beshortened.

1. A magnetic resonance imaging apparatus comprising static magnet fieldgenerating means for generating a static magnet field in a space wherean object to be examined is placed, gradient magnetic field generatingmeans generating gradient magnetic fields in the space in a slicedirection, phase encode direction and readout direction, transmittingmeans for applying RF magnetic field to cause nuclear magnetic resonancein atomic nuclei of a living tissue of the object, receiving means fordetecting echo signals emitted by the nuclear magnetic resonance,control means for controlling the gradient magnetic field generatingmeans, transmitting means and receiving means, signal processing meansfor performing image reconstruction using the echo signals detected bythe receiving means, and display means for displaying the producedimages, wherein the control means performs a three-dimensional sequenceincluding a slice encoding step and a phase encoding step and, uponperforming the sequence, divides sampling points of k-space defined by aslice encode number and phase encode number into two groups and controlsthe gradient magnetic field generating means of the slice direction andthe phase encode direction such that a distance from the k-space originto a sampling point progressively decreases in the measurement of thefirst group and a distance from the k-space origin to a sampling pointprogressively increases in the measurement of the second group.
 2. Themagnetic resonance imaging apparatus of claim 1, wherein the two groupsrespectively belong to either of two regions divided from the k-space.3. The magnetic resonance imaging apparatus of claim 1, wherein the twogroups share the k-space origin and are in a relation of complexconjugate.
 4. The magnetic resonance imaging apparatus of claim 3,wherein the sampling points of the k-space are divided such thatadjacent sampling points belong to different groups.
 5. The magneticresonance imaging apparatus of claim 1, wherein at least one of thegroups includes points which are not sampled.
 6. The magnetic resonanceimaging apparatus of claim 1, wherein an addition set of the two groupsis within a circle inscribed in the k-space.
 7. A data acquisitionmethod of acquiring three-dimensional image data by repeating aplurality of times a step comprising selective excitation of apredetermined region of an object to be examined, application ofencoding gradient magnetic fields at least in two directions andcollection of echo signals emitted from the region while changing theintensities of the gradient magnetic fields, wherein sampling points ina measurement space defined by the intensities of the two directionencoding gradient magnetic fields are divided into the first and secondgroups, the first and second groups are measured successively, in themeasurement, the first group to be measured first is sampled such thatthe distance from the measurement space origin to a sampling pointdecreases in a sampling order and the second group to be measured afteris sampled such that the distance from the measurement space origin to asampling point increases in a sampling order.
 8. A data acquisitionmethod of acquiring three-dimensional image data by repeating aplurality of times a step comprising selective excitation of apredetermined region of an object to be examined, application ofencoding gradient magnetic fields at least in two directions andcollection of echo signals emitted from the region while changing theintensities of the gradient magnetic fields, wherein sampling points ina measurement space defined by the intensities of the two directionencoding gradient magnetic fields are divided into two, the dividedregions are measured successively, in the measurement, the first regionto be measured first is sampled such that the distance from themeasurement space origin to a sampling point decreases in a samplingorder and the second region to be measured after is sampled such thatthe distance from the measurement space origin to a sampling pointincreases in a sampling order.
 9. A data acquisition method of acquiringthree-dimensional image data by repeating a plurality of times a stepcomprising selective excitation of a predetermined region of an objectto be examined, application of encoding gradient magnetic fields atleast in two directions and collection of echo signals emitted from theregion while changing the intensities of the gradient magnetic fields,wherein sampling points in a measurement space defined by theintensities of the two direction encoding gradient magnetic fields aredivided into the first and second groups such that the groups share theorigin and are in a relation of complex conjugate, and adjacent matrixpoints belong to different groups, the first and second groups aremeasured successively, in the measurement, the first group to bemeasured first is sampled such that the distance from the measurementspace origin to a sampling point decreases in a sampling order and thesecond group to be measured after is sampled such that the distance fromthe measurement space origin to a sampling point increases in a samplingorder.
 10. A data acquisition method of claim 7, wherein a part of thesampling points is sampled in one of the two groups and all of thesampling points are sampled in the other group.
 11. A three-dimensionalangiography using a contrast agent comprising the steps of measuring atime from injection of the contrast agent till arrival of the contrastagent at a blood vessel of interest, and performing a data acquisitionaccording to a method of any one of claims 7-9, wherein a time of thebeginning of the measurement of the first group is controlled such thatthe end of the measurement of the first group is coincident with anarrival time of the contrast agent.